The present invention relates generally to the computer art. More particularly, the present invention relates to accessing memory in a personal computer. Specifically, the present invention provides a technique for accessing memory, the physical address of which is located within a reserved range of addresses in a personal computer.
As is well known, a typical personal computer system may have a total memory range capability of one megabyte. Of this total range, a certain smaller range has come to be known as a reserved range. The reserved range is not available for use as system random access memory (RAM), but is reserved for providing the central processing unit (CPU) with direct access to specialized hardware such as video memory or to specialized permanent code stored in read only memory (ROM). The reserved range typically occupies the upper address range of from either 512 or 640 kilobytes up to one megabyte.
Some personal computer systems are, for reasons of speed, constructed with a 32-bit wide data transfer bus. For reasons of economy, such systems use thirty-two 256-kilobit dynamic RAM integrated circuit components, or the equivalent of one megabyte of physical RAM, even though the use of such components provides more physical memory than is necessary to fill the maximum allowable capacity for system RAM. Therefore, since only the first 512 or 640 kilobytes are usable as system RAM, the remainder has heretofore been unusable as being mapped within the reserved range.
Although this typical computer system performs satisfactorily at an acceptable level of cost, it suffers from the consequence of possessing certain physical RAM hardware while being unable to use it. As processing speed and software development costs are related to the amount of usable memory in a computer system, a further consequence is slower speed and greater cost than would be possible if such memory were usable.
Accordingly, a principal object of the present invention is to provide a technique for accessing reserved range RAM which generally overcomes the deficiencies of the prior art.
A more detailed object of the present invention lies in providing a method of using previously unusable RAM which increases the overall system operating speed and reduces attendant software development costs.
A further object lies in providing a method for mapping reserved range RAM that accommodates a system RAM limit of either 512 or 640 kilobytes.
Yet another related object lies in providing a method for mapping reserved range RAM in accordance with an expanded memory specification.